This invention relates generally to manual pressure relief valves and, more particularly, to manual pressure relief valves used in swimming pool filter systems. A pool filter tank is basically a pressure vessel, designed to operate safely over a selected pressure range. Water and compressed air tend to accumulate in the filter tank, and an automatic pressure relief valve is sometimes provided to bleed off compressed air above a selected pressure. Compressed air in a swimming pool filter presents a potential safety hazard. Typically, the pool filter is assembled as two halves clamped together. Compressed air in the filter tank can cause violent separation of the filter halves if they are not properly clamped together or are disassembled when there is pressure in the tank.
Filter tanks are usually provided with a manual air relief valve. Before servicing the filter, maintenance personnel must first open the manual valve, to vent any compressed air in the tank, or to facilitate drainage of the tank. Prior to the present invention, an air relief valve for a swimming pool filter took the form of a petcock, comprising a valve body with an internal valve seat, and a threaded stem with a seal at its end. In its closed position, the stem is engaged in the housing and tightened down, with the valve seal engaging the valve seat. To bleed off the compressed air, the valve stem is unscrewed to lift the seal from the valve seat. The petcock has two significant drawbacks. First, the stem can be unscrewed to the point at which it falls out of the valve housing, and it must be located and replaced before the filter can be operated again. Second, the rate at which air is bled off through the petcock depends on the degree to which the stem is unscrewed. If the petcock is opened only a turn or two, the compressed air will bleed off relatively slowly, and, after a given time, the air pressure in the tank will be higher than if the petcock were opened further. To vent the tank quickly, one usually opens the petcock as far as possible without having it fall out.
It will be appreciated from the foregoing that there is a need for an improved manual air relief valve that avoids the difficulties of the prior art, and provides a more reliable means for venting pool filters and the like. The present invention satisfies this need.